Defect reduction of non-polar and semi-polar iii-nitrides with sidewall lateral epitaxial overgrowth (sleo)

ABSTRACT

A method of reducing threading dislocation densities in non-polar such as a-{11-20} plane and m-{1-100} plane or semi-polar such as {10-1n} plane III-Nitrides by employing lateral epitaxial overgrowth from sidewalls of etched template material through a patterned mask. The method includes depositing a patterned mask on a template material such as a non-polar or semi polar GaN template, etching the template material down to various depths through openings in the mask, and growing non-polar or semi-polar III-Nitride by coalescing laterally from the tops of the sidewalls before the vertically growing material from the trench bottoms reaches the tops of the sidewalls. The coalesced features grow through the openings of the mask, and grow laterally over the dielectric mask until a fully coalesced continuous film is achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. Section 120 ofco-pending and commonly-assigned U.S. Utility patent spplication Ser.No. 12/041,398, filed on Mar. 3, 2008, by Bilge M. Imer, James S. Speckand Steven P. Denbaars, entitled “DEFECT REDUCTION OF NON-POLAR ANDSEMI-POLAR III-NITRIDES WITH SIDEWALL LATERAL EPITAXIAL OVERGROWTH(SLEO),” attorneys' docket no. 30794.135-US-C1 (2005-565-3), whichapplication is a continuation under 35 U.S.C. Section 120 of co-pendingand commonly-assigned U.S. Utility patent application Ser. No.11/444,084, filed on May 31, 2006, by Bilge M. Imer, James S. Speck andSteven P. Denbaars, entitled “DEFECT REDUCTION OF NON-POLAR ANDSEMI-POLAR III-NITRIDES WITH SIDEWALL LATERAL EPITAXIAL OVERGROWTH(SLEO),” attorneys' docket no. 30794.135-US-U1 (2005-565-2), whichapplication claims the benefit under 35 U.S.C. Section 119(e) ofco-pending and commonly-assigned U.S. Provisional Patent ApplicationSer. No. 60/685,952, filed on May 31, 2005, by Bilge M. Imer, James S.Speck and Steven P. Denbaars, entitled “DEFECT REDUCTION OF NON-POLARGALLIUM NITRIDE WITH SINGLE-STEP SIDEWALL LATERAL EPITAXIAL OVERGROWTH,”attorneys' docket no. 30794.135-US-P1 (2005-565-1),

all of which applications are incorporated by reference herein.

This application is related to the following co-pending andcommonly-assigned applications:

U.S. Utility patent application Ser. No. 11/444,083, filed on May 31,2006, now U.S. Pat. No. 7,338,828, issued Mar. 4, 2008, by Bilge M.Imer, James S. Speck and Steven P. DenBaars, entitled “GROWTH OF PLANARNON-POLAR {1-1 0 0} M-PLANE GALLIUM NITRIDE WITH METALORGANIC CHEMICALVAPOR DEPOSITION (MOCVD),” attorneys docket number 30794.136-US-U1(2005-566), which application claims the benefit under 35 U.S.C. Section119(e) of U.S. Provisional Patent Application Ser. No. 60/685,908, filedon May 31, 2005 by Bilge M. Imer, James S. Speck and Steven P. DenBaars,entitled “GROWTH OF PLANAR NON-POLAR {1-100} M-PLANE GALLIUM NITRIDEWITH METALORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD),” attorneys docketnumber 30794.136-US-P1 (2005-566);

all of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to defect reduction of non-polar andsemi-polar III-Nitrides with sidewall lateral epitaxial overgrowth(SLEO).

2. Description of the Related Art

Gallium nitride (GaN) and its ternary and quaternary compounds are primecandidates for fabrication of visible and ultraviolet high-power andhigh-performance optoelectronic devices and electronic devices. Thesedevices are typically grown epitaxially as thin films by growthtechniques including molecular beam epitaxy (MBE), metalorganic chemicalvapor deposition (MOCVD), or hydride vapor phase epitaxy (HVPE). Theselection of substrates is critical for determining the III-Nitridegrowth direction. Some of the most widely used substrates for nitridegrowth include SiC, Al₂O₃, and LiAlO₂. Various crystallographicorientations of these substrates are commercially available which causea-plane, m-plane, or c-plane growth of GaN.

FIGS. 1( a) and 1(b) are schematics of crystallographic directions andplanes of interest in hexagonal GaN. Specifically, these schematics showthe different crystallographic growth directions and also the planes ofinterest in the hexagonal wurtzite GaN structure, wherein FIG. 1( a)shows the crystallographic directions a1, a2, a3, c, <10-10> and<11-20>, and FIG. 1( b) shows planes a (11-20), m (10-10) and r (10-12).The fill patterns of FIG. 1( b) are intended to illustrate the planes ofinterest, but do not represent the materials of the structure.

It is relatively easy to grow c-plane GaN due to its large growth window(pressure, temperature and precursor flows) and its stability.Therefore, nearly all GaN-based devices are grown along the polarc-axis. However, as a result of c-plane growth, each material layersuffers from separation of electrons and holes to opposite faces of thelayers. Furthermore, strain at the interfaces between adjacent layersgives rise to piezoelectric polarization, causing further chargeseparation. FIGS. 2( a) and 2(b), which are schematics of band bendingand electron hole separation as a result of polarization, show thiseffect, wherein FIG. 2( a) is a graph of energy (eV) vs. depth (nm) andrepresents a c-plane quantum well, while FIG. 2( b) is a graph of energy(eV) vs. depth (nm) and represents a non-polar quantum well.

Such polarization effects decrease the likelihood of electrons and holesrecombining, causing the device to perform poorly. One possible approachfor eliminating piezoelectric polarization effects in GaN optoelectronicdevices is to grow the devices on non-polar planes such as a-{11-20} andm-{1-100} plane. Such planes contain equal numbers of Ga and N atoms andare charge-neutral.

Another reason why GaN materials perform poorly is the presence ofdefects due to lack of a lattice matched substrate. Bulk crystals of GaNare not widely available so it is not possible to simply cut a crystalto present a surface for subsequent device regrowth. All GaN films areinitially grown heteroepitaxially, i.e., on foreign substrates that havea lattice mismatch to GaN.

There is an ever-increasing effort to reduce the dislocation density inGaN films in order to improve device performance. The two predominanttypes of extended defects of concern are threading dislocations andstacking faults. The primary means of achieving reduced dislocation andstacking fault densities in polar c-plane GaN films is the use of avariety of lateral overgrowth techniques, including single step anddouble step lateral epitaxial overgrowth (LEO, ELO, or ELOG), selectivearea epitaxy, cantilever and pendeo-epitaxy. The essence of theseprocesses is to block (by means of a mask) or discourage dislocationsfrom propagating perpendicular to the film surface by favoring lateralgrowth over vertical growth. These dislocation-reduction techniques havebeen extensively developed for c-plane GaN growth by HVPE and MOCVD.

The present invention is the first-ever successful execution of sidewalllateral epitaxial overgrowth (SLEO) of non-polar a-plane and m-plane GaNby any growth technique. Prior to the invention described herein, SLEOof a-plane and/or m-plane GaN had not been demonstrated.

SUMMARY OF THE INVENTION

The general purpose of the present invention is to create high quality(minimum defect density) non-polar a-{11-20} and m-{1-100} plane andsemi-polar {10-1n} plane III-Nitride material by employing lateralovergrowth from sidewalls of etched nitride material through adielectric mask. The method includes depositing a patterned mask onnon-polar or semi-polar III-Nitride template, etching the templatematerial down to various depths through openings in the mask, andregrowing the non-polar or semi-polar epitaxial film by coalescinglaterally from the tops of the sidewalls before the vertically growingmaterial from the trench bottoms reaches the surface. The coalescedfeatures grow through the openings of the mask, and grow laterally overthe dielectric mask until a fully coalesced continuous film is achieved.

These planar non-polar materials grown heteroepitaxially, such as a-GaNon top of r-Al₂O₃, contain dislocation densities of ˜10¹⁰ cm⁻² andstacking fault densities of 3.8×10⁵ cm⁻¹ (aligned perpendicular to thec-axis) throughout the film. By using single step lateral epitaxialovergrowth, dislocation densities can be reduced down to ˜10⁷-10 ⁹ cm⁻²and stacking faults are localized only on the nitrogen faces. With thepresent invention, using sidewall lateral epitaxial overgrowth,dislocation densities can be reduced down to even lower values byeliminating defects not only in the overgrown regions but also in thewindow regions. Also, by favoring gallium (Ga) face growth and limitingnitrogen (N) face growth stacking fault densities can be made orders ofmagnitude lower.

The present invention also takes advantage of the orientation ofnon-polar III-Nitrides to eliminate polarization fields. As a result,with the material produced by utilizing this invention, deviceimprovements such as longer lifetimes, less leakage current, moreefficient doping and higher output efficiency will be possible. Inaddition, a thick non-polar and semi-polar nitride free-standingsubstrate, which is needed to solve the lattice mismatch issue, can beproduced over this excellent material by various methods.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIGS. 1( a) and 1(b) are schematics of crystallographic directions andplanes of interest in hexagonal GaN.

FIGS. 2( a) and 2(b) are schematics of band bending and electron holeseparation as a result of polarization.

FIG. 3 is a flowchart, including associated schematics, that illustratesthe three SLEO processing steps and the three stages of regrowth.

FIG. 4( a) is a scanning electron microscopy image of SLEO materialgrown by MOCVD in the first growth stage, and FIG. 4( b) is a schematicthat illustrates the first stage of sidewall growth from the tops of theGaN pillar sides which blocks growth from the trench bottoms.

FIG. 5( a) is a scanning electron microscopy image of a coalesced SLEOmaterial and FIG. 5( b) is a schematic that further illustrates thecoalesced SLEO material.

FIGS. 6( a), 6(b) and 6(c) are atomic force microscopy images, and x-raydiffraction Full Width Half Maximum values that illustrate the dramaticimprovement in film quality by making side by side comparison of planarnon-polar planar, single step LEO, and SLEO GaN.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings that form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

The growth of (Ga, In, Al, B) N materials in the polar [0001]c-directioncauses lower performance in optical devices due to polarization fieldscausing charge separation along the primary conduction direction.Therefore, research has recently been conducted on non-polar directiongrowth along the a-[11-20] and m-[1-100] directions of these materialsto eliminate such effects and improve device performance significantly.

Another problem that is common to polar, semi-polar and non-polarIII-Nitride materials is high defect densities, the most common of whichare dislocations and stacking faults. Dislocations arise as a result oflattice mismatch in heteroepitaxial growth due to a lack of properIII-Nitride substrates, and stacking faults form because of disorder ofatomic stacking during growth, which is, for example, predominant on thenitrogen face sidewall during a-plane GaN growth. With the presentinvention, the presence of these stacking faults can be minimized byfavoring Ga face growth and limiting N face growth.

Dislocation densities in directly grown (Ga, In, Al, B)N materials arequite high. High-performance devices could be achieved by reducing orideally eliminating these defects accompanied by the use of non-polarmaterials. Such defects have been reduced by various methods involvingLEO in polar and non-polar GaN over the years. The essence of theseprocesses is to block or discourage dislocations from propagatingperpendicular to the film surface by favoring lateral growth oververtical growth. Any LEO method involves blocking of defective materialwith the use of a mask deposited on the surface. Single-step LEOinvolves only one mask patterning and regrowth step, so it is simple toprocess and grow, but the results are not as effective as double-stepLEO at defect reduction. Although double LEO is effective in defectreduction, it takes twice the amount of processing and growth efforts ascompared to single step LEO, as the name implies. As a result, none ofthese methods have been both convenient and effective enough at the sametime until now. With the use of SLEO in the present invention, it ispossible to eliminate these defects in non-polar or semi-polar nitridesas effectively as double-step LEO by using as simple processing andgrowth methods as single-step LEO does. This invention nucleates on andgrows from the tops of etched pillar sidewalls of non-polar orsemi-polar nitride material, and coalesces the tops of the adjacentpillar sidewalls before the defective material from the heteroepitaxialinterface (at the bottom of the trenches) reaches the top.

The present invention improves the materials' device performance in twoways: (1) by utilizing the natural structural advantage of non-polarmaterial, a-{11-20} and m-{1-100} plane or semi-polar {10-1n}planeIII-Nitride materials, to eliminate or reduce polarization effects, and(2) by eliminating defects effectively while employing a unique,reproducible, simple, and efficient process and growth method.

Technical Description

The present invention reduces threading dislocation densities innon-polar and semi-polar nitrides by employing LEO from sidewalls ofetched nitride material through a dielectric mask. As described earlier,stacking faults reside on the N face, one of the vertically orientedfaces. This invention also decreases stacking fault densities with ananisotropy factor, i.e., by encouraging higher growth rates on theGa-(0001) face and limiting the N-(000-1) face growth rates. Byutilizing various growth conditions and processing methods, the presentinvention has demonstrated lateral growth and coalescence of non-polarGaN from sidewalls, and up and over the dielectric mask.

FIG. 3 is a flowchart, including associated schematics, that illustratesa method of reducing threading dislocation densities in non-polar orsemi-polar III-Nitride material, employing LEO of non-polar orsemi-polar III-Nitride material from sidewalls of etched templatematerial through a patterned mask.

In this embodiment, the method comprises three SLEO processing stepslabeled as A, B and C, and three stages of growth or regrowth labeled asD, E and F.

Step A—A template material (1) is formed on an appropriate substrate(2). The template material (1) may comprise a non-polar or semi-polarnitride epitaxial film, such as {11-20} or {1-100} or {10-1n} plane GaN,deposited on the appropriate substrate (2).

Step B—A patterned mask (3) is formed on the template material (1). Thepatterned mask (3) may be a dielectric mask and may be deposited on thetemplate material (1) by one of a variety of means. The patterned mask(3) may comprise one or more openings (4), wherein the openings (4)comprise apertures or stripes allowing access to the underlying templatematerial (1).

Step C—The template material (1) is etched through the openings (4) inthe patterned mask (3) to form one or more trenches (5) or pillars (6)in the template material (1), wherein the trenches (5) or pillars definesidewalls (7). The orientation of the openings (4) is aligned in a waythat creates planar sidewalls (7) in subsequent lateral growth stepswhen the template material (1) is etched through the openings (4) in thepatterned mask (3).

Following Step C, the template material (1) is loaded into a reactor forthe growth stages.

Step D—A first growth stage “SIDE” is performed, wherein non-polar orsemi-polar III-Nitride material, which may comprise {11-20} or {1-100}or {10-1n} plane GaN, is grown, first by growing and coalescing thenon-polar or semi-polar III-Nitride material laterally from the tops (8)of the sidewalls (7) (as indicated by the arrows), before the non-polaror semi-polar III-Nitride material vertically growing from the bottoms(9) of the trenches (5) reaches the tops (8) of the sidewalls (7).Preferably, the non-polar or semi-polar III-Nitride material grows onlyfrom the regions of exposed template material (1), but not on thepatterned mask (3). Specifically, the non-polar or semi-polarIII-Nitride material nucleates on and grows from both the tops (8) ofthe sidewalls (7) and the bottoms (9) of the trenches (5). The non-polaror semi-polar III-Nitride material vertically growing from the bottoms(9) of the trenches (5) may comprise defective material from aheteroepitaxial interface (10). Coalescing the non-polar or semi-polarIII-Nitride material growing laterally from the tops (8) of thesidewalls (7) may block the defected material vertically growing fromthe bottoms (9) of the trenches (5).

Step E—A second growth stage “UP” is performed, wherein the non-polar orsemi-polar III-Nitride material (11) grows vertically up through theopenings (4) (as indicated by the arrows), after the first growth stageor coalescing step.

Step F—A third growth stage “OVER” is performed, wherein the non-polaror semi-polar III-Nitride material (12) grows laterally over thepatterned mask (3) (as indicated by the arrows) to form an overgrownnon-polar or semi-polar III-Nitride material. This growth may continueuntil the overgrown material (12) forms a fully coalesced continuousfilm, or even if it remains an uncoalesced overgrowth.

The result of the method of FIG. 3 is a device, or a free standingwafer, or a substrate, or a reduced defect density template, fabricatedusing the method.

Note that the template material (1) may have a thickness comparable orscaled relative to dimensions of the openings (4) in the patterned mask(3), in order to compensate for competing lateral to vertical growthrates. In this regard, the dimensions are the width of the openings (4),or the dimensions along the lateral direction in which the tops (8) ofthe sidewalls (7) are coalescing.

For example, to compensate, the template material (1) thickness ischosen with respect to the width dimension of the openings (4), whichwill form a trench (5) after etching, having a separation such that thetops (9) of the sidewalls (7) coalesce before growth from the bottoms(9) of the trenches (5) reach the tops (8) of the sidewalls (7).

This means that, if the lateral and vertical growth rates arecomparable, then the dimensions of the openings (4) should be less thanthe thickness of the template material (1). Alternatively, if thelateral growth rate is faster than the vertical growth rate, then thedimensions of the openings (4) can be larger than the thickness of thetemplate material (1).

Similarly, the etching may be to one or more etch depths comparable orscaled relative to the dimensions of the openings (4), in order for thetops (8) of the sidewalls (7) to coalesce before the defected non-polaror semi-polar III-Nitride material growing from the bottoms (8) of thetrenches (5) reaches the tops (8) of the sidewalls (7).

Further, growth from the bottoms (9) of the trenches (5) may beprevented by etching to the substrate (2), or by depositing anadditional mask (3) on the bottoms (9) of the trenches (5). At leastSteps A and C may comprise growth using a lateral overgrowth technique.

To control a lateral and vertical growth rate during the first (D),second (E) or third (F) growth stages illustrated in FIG. 3, growthconditions for non-polar a-{11-20} plane gallium nitride films arespecified by: a temperature in a range of 1000-1250 ° C., a reactorpressure in a range of 20-760 Ton, and a V/III ratio in a range of100-3500, wherein during at least one of the first, second or thirdgrowth stages the conditions are such that the lateral growth rate isgreater than the vertical growth rate. These conditions may vary fromreactor to reactor and also from one growth method to another.

By employing the present invention, the dislocation density is reducedby eliminating or reducing defects in the overgrown non-polar orsemi-polar III-Nitride material over the mask (3) and coalesced openings(4) (window) regions. In this regard, dislocation densities may bereduced at least one order of magnitude lower than that achieved innon-polar or semi-polar III-Nitride material using conventional singlestep LEO.

Also, with proper growth conditions, the present invention decreasesstacking fault densities with an anisotropy factor, by encouraginghigher growth rates on a gallium (Ga) face of the non-polar III-Nitridematerial and limiting growth rates on a nitrogen (N) face of thenon-polar III-Nitride material, thereby reducing stacking faultdensities at least one order of magnitude lower than that achieved innon-polar nitride material or achieved using conventional single-stepLEO, and confining stacking faults to the N faces.

Finally, the present invention may be performed using any epitaxialmaterial that can be grown using a lateral overgrowth technique. Thus,the epitaxial material may comprise, but is not limited to, non-polar orsemi-polar III-Nitride material such as non-polar {11-20} or {1-100}plane GaN or semi-polar {10-1n} plane GaN. Thus, the template materialmay comprise any material from which nucleation and growth of theepitaxial material is achievable.

Experimental Results

As an example, 3 μm thick non-polar a-plane or m-plane GaN film isdeposited on an r-plane Al₂O₃ substrate using a low-temperature GaNnucleation layer by MOCVD to form a template. Alternatively, the filmcould be deposited on m-plane SiC using an AN nucleation layer. A 1 μmthick SiO₂ film is deposited on this template by plasma-enhancedchemical vapor deposition (PECVD). Conventional photolithography is usedto pattern a photoresist mask layer comprised of 8 μm-wide stripesseparated by 2 μm-wide openings. The stripe orientation is chosen as<1-100> for a-GaN and as <11-20> for m-GaN. The GaN template thicknessis chosen with respect to the width of the mask window, which will forma trench with a separation such that the tops of the GaN sidewallscoalesce before the bottoms of the trenches reach the top. The patternedmask is then dry etched using inductively coupled plasma (ICP) etchingto get vertical SiO₂ sidewalls, completely etching the exposed SiO₂away. The remaining photoresist is removed with stripper, andsubsequently the samples are cleaned with solvent. 3 μm thick non-polarGaN exposed through the SiO₂ openings is etched more than 2 μm down byusing reactive ion etching (RIE). The sample is treated with a finalsolvent clean before regrowth. The wafer, which now consists of anetched non-polar GaN template that is patterned with 8 μm-wide SiO₂stripes separated by 2 μm-wide openings, is loaded into the MOCVDreactor for regrowth. During this specific regrowth, relatively highgrowth temperatures are used at low pressures and various V/III ratiosto enhance lateral growth. The V/III ratio is changed to control thelateral and vertical growth rates comparably at different stages ofgrowth. During the growth process, GaN nucleates on the GaN sidewallsand exposed GaN material (or Al₂O₃ or SiC) at the bottom of thetrenches, and coalesces from the tops of the etched GaN sidewalls, andgrows out through the mask openings up and over the SiO₂ mask. The filmthen grows laterally over the SiO₂ mask, eventually coalescing withneighboring GaN stripes.

FIG. 4( a) is a scanning electron microscopy image of SLEO materialgrown by MOCVD in the first growth stage, and FIG. 4( b) is a schematicthat illustrates the first stage of sidewall growth from the tops of theGaN pillar sides which blocks growth from the trench bottoms.

An example of a coalesced a-plane GaN film grown by SLEO with MOCVD isshown in FIGS. 5( a) and 5(b) in a cross-section view, wherein FIG. 5(a) is a scanning electron microscopy image of a coalesced SLEO materialand FIG. 5( b) is a schematic that further illustrates the coalescedSLEO material.

Regrowth in this example proceeds first by coalescing non-polar GaNlaterally from the tops of the etched sidewalls (Step D—first growthstage), before the defective material growing from the trench bottomreaches the tops of the sidewalls, as described above. Then, thecoalesced material grows up from the opening (Step E—second growthstage) and over the 8 μm-wide mask stripe (Step F—third growth stage),until adjacent stripes of non-polar material coalesce forming acontinuous a-GaN film with low defect density. By utilizing sidewallgrowth, threading dislocations are blocked, so coalesced films havelower dislocation densities not only in the overgrown regions but alsoin the window regions. By utilizing higher Ga-face horizontal growthrates, the stacking fault density is reduced and limited only to areasof N-face overgrowth.

A completely coalesced film of such SLEO material was achieved usingMOCVD, as shown in FIGS. 5( a) and 5(b). A stripe of non-polar GaN wasgrown through a 2 μm-wide opening (comprising a window) in a SiO₂ mask,which was patterned parallel to <1-100> to achieve planar sidewalls insubsequent lateral growth steps. The non-polar GaN was subsequentlygrown over the 8 μm-wide masked region defined by a mask stripe.

Possible Modifications and Variations

The preferred embodiment has described a LEO process from the etchedsidewalls of a non-polar or semi-polar III-Nitride template. Alternativeappropriate substrate materials, on which the non-polar or semi-polarIII-Nitride or GaN template could be formed include but are not limitedto a- and m-plane SiC or r-plane Al₂O₃. The template material to use asa base for the sidewall growth process can be any non-polar orsemi-polar III-Nitride template material including but not limited toGaN, AlN, AlGaN, and InGaN with various thicknesses and crystallographicorientations. This material can be formed by any means using MOCVD orHVPE or any other variety of methods. To grow such template materialdifferent nucleation layers including GaN and AlN can be used. A varietyof mask materials, including dielectric, and geometries with variousaperture or opening spacing, sizes and dimensions may be used. Maskdeposition methods with different mask thicknesses, and mask patterningtechnique with various orientations may be used in practice of thisinvention without significantly altering the results. Many alternativeetching methods, including but not limited to wet and dry etchingtechniques, can be used while etching the mask and/or the templatematerial. The etch depth of the template material can be varied as longas the material growing laterally from the sidewalls coalesces andblocks the defective material growing vertically from the trenchbottoms. Etching of the substrate can be included in the process toensure growth only from the sidewalls. The one or more trenches formedby the etching may have a variety of shapes, comprising U shaped or Vshaped grooves, holes or pits.

Another possible variation could be that after etching the III-Nitridematerial as described above, an additional mask may be deposited on thebottom of the trenches to allow regrowth from the sidewalls only.

The growth parameters required for the lateral overgrowth of non-polaror semi-polar III-Nitride from the sidewalls will vary from reactor toreactor. Such variations do not fundamentally alter the general practiceof this invention. Although it is desirable, final coalescence of thefilm over the mask is not a requirement for the practice of thisinvention. Therefore, this disclosure applies to both coalesced anduncoalesced laterally overgrown non-polar or semi-polar III-Nitridefilms from sidewalls.

The invention described herein, and all its possible modifications, canbe applied multiple times by repeating the SLEO process after achievingcoalescence, one layer over another layer, thereby creating a multi-stepSLEO process to decrease defect densities even further.

This invention can be practiced with any kind of growth method includingbut not limited to metalorganic chemical vapor deposition (MOCVD), andHydride Vapor Phase Epitaxy (HVPE), and molecular beam epitaxy (MBE), orthe combination of any of these growth methods at various stages of SLEOprocessing and growth.

Advantages and Improvements

The present invention is the first-ever successful execution of SLEO ofnon-polar GaN. It is now possible to reduce the presence of dislocationsmost effectively in the simplest possible way in non-polar or semi-polarIII-Nitride materials, while preventing polarization effects in theresulting devices.

As an example, FIGS. 6( a), 6(b) and 6(c) are atomic force microscopyimages, and x-ray diffraction FWHM values that illustrate the dramaticimprovement in film quality by making side by side comparison of planarnon-polar planar, single-step LEO, and SLEO GaN. FIG. 6( a) shows planara-GaN with an rms of ˜6.0 nm with an on-axis FWHM of 0.29° (110) and anoff-axis FWHM of 0.46° (101). FIG. 6( b) shows a single-step LEO a-GaNwith an rms of ˜5.822 (0.467) nm with an on-axis FWHM of 0.17° and anoff-axis FWHM of 0.27°. FIG. 6( c) shows a SLEO a-GaN with an rms of˜0.620 (0.499) nm with an on-axis FWHM of 0.082° and an off-axis FWHM of0.114°.

Planar non-polar GaN films have been reported to have high threadingdislocation densities of ˜10¹⁰ cm⁻² and stacking fault densities of3.8×10⁵ cm⁻¹ (perpendicular to c-axis, on N face). These values arereduced by one or two orders of magnitude by using single step lateralepitaxial growth by HVPE or MOCVD. Yet with the present invention, thedensity of these dislocations and stacking faults was reduced furtherdown to ˜10⁶-10⁷ cm⁻² and 10³-10⁴ cm⁻¹, respectively.

Previous to this discovery, reduced defect density with a fullycoalesced smooth non-polar GaN film was tried by means of single-stepLEO and double-step LEO methods. But neither of these methods wassuccessful in getting fully coalesced dislocation free, reproduciblematerial in the most time and energy-efficient way. Even thoughdouble-step LEO was able to reduce the majority of defects, it requiredlarge amounts of time, energy, and resources. Conventional single-stepLEO, even though it was simple (involving only one set of processing andgrowth steps), could not eliminate the majority of the defects.

The present discovery, combining the advantages of these two methods,allows significant defect reduction by leaving most areas defect-freewhile involving only one processing and growth step. In other words,this invention allows having double-step LEO results with single-stepLEO efforts. Such effective defect reduction and elimination ofpolarization field at the same time will provide for breakthroughimprovements in the electronic, optoelectronic, and electromechanicaldevices that are subsequently grown over this material.

A previous report similar to sidewall lateral overgrowth (SLEO) of GaNby MOCVD is known as pendeo-epitaxy. This technique has beendemonstrated only for polar c-plane GaN growth. And, it also hasfundamental differences in terms of processing and growth. For example,the substrate, relatively expensive SiC, is used as a “pseudo” mask,meaning that the growth takes place selectively only at the sidewallsand not on the substrate. As a result, the material has to be etcheddown to the substrate and also the etching process should be continuedinto the substrate until a certain depth. Consequently, the growth doesnot initiate through open windows. Therefore, there is no variableinvolved during growth to coalesce tops of the sidewalls through theopen windows before the vertically grown material from the bottom of thetrenches reaches the tops of the sidewalls. The lateral growth involvesthe nucleation on and growth from the whole etched sidewall. The mainfocus is the growth of the whole pillar.

Another similar study, lateral overgrowth from trenches (LOFT),suggested growing GaN from the trenches by only exposing the sidewallsafter depositing SiO₂ mask to the top and the bottom of the pillars.This was demonstrated only for polar c-GaN.

Presently, GaN films must be grown heteroepitaxially due to theunavailability of bulk crystals, and no perfectly lattice-matchedsubstrates exist for this growth process. As a result, the presentinvention also produces an excellent material base to grow free standingGaN substrate for eventual homoepitaxial growth.

REFERENCES

The following references are incorporated by reference herein:

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2. Y. Chen, R. Schneider, Y. Wang, “Dislocation reduction in GaN thinfilms via lateral overgrowth from trenches”, Appl. Phys. Letters., 75(14) 2062 (1999).

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CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching, such asadditional adjustments to the process described herein, withoutfundamentally deviating from the essence of the present invention. It isintended that the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A device having a structure that includes a lateral epitaxial overgrowth from sidewalls of a patterned substrate, wherein the lateral epitaxial overgrowth is grown in a semi-polar or non-polar direction and has a top surface that is semi-polar or non-polar, the lateral epitaxial overgrowth comprising a semi-polar or non-polar III-nitride film.
 2. The device of claim 1, wherein the lateral epitaxial overgrowth is grown in the non-polar direction and has the top surface that is non-polar, the lateral epitaxial overgrowth comprising the non-polar III-nitride film.
 3. The device of claim 2, wherein the non-polar III-nitride film comprises a threading dislocation density of 10⁶ cm⁻² or less.
 4. The device of claim 2, wherein the non-polar III-nitride film comprises a stacking fault density of 10³ cm⁻¹ or less.
 5. The device of claim 2, wherein the non-polar III-nitride film comprises a stacking fault density of 10⁴ cm⁻¹ or less.
 6. The device of claim 2, wherein the non-polar III-nitride film is substantially defect free.
 7. The device of claim 1, wherein the lateral epitaxial overgrowth is grown in the semi-polar direction and has the top surface that is semi-polar, the lateral epitaxial overgrowth comprising the semi-polar III-nitride film.
 8. The device of claim 7, wherein the semi-polar III-nitride film comprises a threading dislocation density of 10⁶ cm² or less.
 9. The device of claim 7, wherein the semi-polar III-nitride film comprises a stacking fault density of 10³ cm⁻¹ or less.
 10. The device of claim 7, wherein the semi-polar III-nitride film comprises a stacking fault density of 10⁴ cm⁻¹ or less.
 11. The device of claim 7, wherein the semi-polar III-nitride film is substantially defect free.
 12. The device of claim 1, wherein the patterned substrate is an initial non-polar or semi-polar III-nitride layer and a top surface of the initial non-polar or semi-polar III-nitride layer is an epitaxially grown non-polar or semi-polar surface.
 13. The device of claim 1, wherein the device is an electronic, optoelectronic, or electromechanical device.
 14. The device of claim 1, wherein the semi-polar or non-polar III-nitride film is a fully coalesced planar film.
 15. The device of claim 1, wherein the semi-polar or non-polar III-nitride film is a planar a-plane, m-plane, or (10-1n) plane film.
 16. A method of fabricating a non-polar or semi-polar III-nitride device, comprising: performing a lateral epitaxial overgrowth from sidewalls of a patterned substrate, wherein the lateral epitaxial overgrowth is grown in a semi-polar or non-polar direction and has a top surface that is semi-polar or non-polar, the lateral epitaxial overgrowth comprising a semi-polar or non-polar III-nitride film, and the device having a structure including the lateral epitaxial overgrowth.
 17. The method of claim 16, wherein the patterned substrate is an initial non-polar or semi-polar III-nitride layer and a top surface of the initial non-polar or semi-polar III-nitride layer is an epitaxially grown non-polar or semi-polar surface.
 18. The method of claim 16, wherein the lateral epitaxial overgrowth is grown by Metal Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase Epitaxy (HVPE), or Molecular Beam Epitaxy (MBE).
 19. A non-polar or semi-polar III-nitride device, comprising: an initial non-polar or semi-polar III-nitride layer grown on a non-III-nitride substrate, wherein a top surface of the initial non-polar or semi-polar III-nitride layer is an epitaxially grown non-polar or semi-polar surface; and a subsequent non-polar or semi-polar III-nitride layer grown on the initial non-polar or semi-polar III-Nitride layer, comprising a threading dislocation density of no more than 10⁷ cm⁻², wherein the non-polar or semi-polar III-nitride device is grown on a non-polar or semi-polar top surface of the subsequent non-polar or semi-polar III-nitride layer.
 20. The device of claim 19, wherein the initial non-polar III-nitride layer has a top surface that is the non-polar surface and the non-polar III-nitride device is grown on the non-polar top surface of the subsequent non-polar III-nitride layer.
 21. The device of claim 19, wherein the initial semi-polar III-nitride layer has a top surface that is the semi-polar surface and the semi-polar III-nitride device is grown on the semi-polar top surface of the subsequent semi-polar III-nitride layer. 